270 volt DC power supplies have been proposed for airframes. A three phase alternator is driven by each propulsion engine of the airframe to generate variable frequency three phase AC. The three phase AC is rectified to produce 270 volt DC which becomes the main potential for powering appliances in the airframe with inverters being used to generate AC where necessary. The proposed DC power supplies utilize contactors to connect and disconnect the 270 volt DC produced by each propulsion engine to an electrical load. Contactors of the aforementioned type will be required to switch 270 volts at high current levels typically of 100 amperes or more. Furthermore, the DC output power produced by each propulsion engine may be selectively coupled in series with an electrical load or coupled in parallel with the DC output potential produced by another propulsion engine for driving the load in parallel. A bus control unit will control the connection of a main power bus selectively to the electrical load(s) and to the DC potential generated by generators driven by each of the propulsion engines.
U.S. Pat. No. 4,772,809 discloses a switching circuit relay for preventing arcing when connecting and disconnecting an AC power source from an electrical load. FIG. 4 discloses an embodiment in which a relay Xl is in series with a parallel combination of a relay Y2 and a triac T. Because the potential is AC no consideration is given to disconnecting the load from the AC source with respect to a non-ground potential. Fault protection for interrupting connection of the load to the AC power source is not provided.
Arc suppression circuits are known which utilize a combination of relays and semiconductor switches. See U.S. Pat. Nos. 4,438,472, 4,636,906, 4,636,907 and 4,760,483.